Horizontal form fill seal wrapping machines are well known in the art and are generally characterized by a horizontal conveyor that transports discrete units of product through a forming head over which film is drawn to wrap the product in a continuous tube made by a bottom seal running longitudinally along the overlapped edges of the film after it has been formed into a tube. This tube containing spaced packages is then transversely sealed and severed to discharge units of product enveloped in a bag or envelope of the wrapping material having a longitudinal seal and two end seals. If shrink film is employed this package is generally discharged to a shrink tunnel where the film is heat shrunk to produce a tight-skinned package.
The general horizontal form fill seal machine of the prior art uses a flighted infeed conveyor to present product to the forming head or to a conveyor which conveys product to the tube forming head in uniformly spaced sequence. Other versions of such machines employ a ram for conveying the product one unit at a time which is fed to a position in front of the ram and delivered to the film forming mechanism on each stroke of the ram.
Prior art packaging machines of the general type here under consideration have a number of disadvantages. If such machines are operated at high speed they become fairly complex and high in cost. Those which use a flighted infeed conveyor must be accurately timed and the speed control of the infeed conveyor has to be continuously variable in order to produce exactly the right spacing of product going through the machine. Usually since product is not available in uniform condition for feeding to the infeed conveyor, some type of product placement system or magazine or accumulating device must be employed. Such machines also generally run best with a continual flow of product and when properly installed and operating have the advantage of being able to run at substantial high speed (60 to 100 packages per minute or faster).
The prior art machines which use a ram to thrust the product into the film provide a simple transport mechanism and generally produce a tighter package because the package is thrust against the end seal made by severing the preceding package. This feature also results in a disadvantage since the infeeding package pressing against the newly formed transverse seal before it has had adequate time to completely cool can burst the seal or form a weakened seal that will burst in the shrink tunnel. This problem is particularly pronounced when polyethylene film is used since it takes much more time to set up than PVC or polyolefin shrink film. Also to develop package speeds of 30 packages per minute or more the ram has to operate with substantial acceleration and the resultant impact or force on the product is a disadvantage particularly with stacked, loosely packed or fragile articles. These problems, particularly the seal breakage problem, result in a practical barrier to higher machine speeds.
The speed with which the ram advances the product along the longitudinal sealing shoe is so high that it presents problems in forming an adequate longitudinal seal for the overlapped edges of the film that form the tube.
Ram type machines also have difficulty with high profile products since the transverse seal is formed close to the forming head and it is difficult to draw the tube down if the height of the product and hence the spacing between the top and bottom portions of the tube is large. If a longer ram stroke is used to remove the transverse seal downstream from the forming head the dynamics of ram motion require that the machine be slowed down.
Machines have been built in the past which use differential speed conveyors to transport product and develop the desired spacing between sequential units of product. Thus if an infeed conveyor is running at a given speed and it deposits product on an intermediate conveyor with differential speed control available between the linear speed of the two conveyors it is possible to adjust the relative speeds so that a spacing develops between sequential product as it is transported through the film forming head thus providing an adequate free space in the tube containing sequential product for making the transverse seals. Such machines require adequate control of the differential speeds of the conveyors, but even if this control is achieved, the shape, length, and weight distribution of each individual package must be uniformly the same, or the transfer of products from one conveyor to the next and thus the package spacing will vary. In reality, many products do not have this uniformity because of irregular weight distribution (natural products such as produce or loose or randomly picked products) or imperfect containers (such as freshly formed boxes with bottoms not perfectly flat). Even in a machine using differential speeds to separate abutting products and with a stop-start conveyor control sequence used to further control the spacing of packages, the speed variation and non-uniform product transfer characteristics previously described make it virtually impossible to obtain the accurate control of product spacing. Since the final appearance of the package is often dependent upon the amount of excess film left on the package ends, precise spacing is frequently essential for good final package appearance.
Machines of the prior art generally have requirements that restrict the input feeding of product in some manner. They cannot accurately space product in the tube of film simply and automatically while accepting randomly fed product including abutting adjacent product or interspersed product of substantially different lengths, shapes and weight distribution while running continuously at moderately high speeds up to 60 packages per minute or higher unless a magazine or some other type of accumulating device is added to the infeed.